Bone formation arises by a transformation of connective tissue, and may be preceded either by the laying down of cartilage, or by direct transformation of fibrous tissue. In bones which have been preformed in cartilage, a portion of the cartilage persists throughout the period of growth as a cartilage plate. This cartilage continues to grow, and is constantly replaced by bone, resulting in lengthwise bone growth. The cartilage with its surrounding tissues, also contributing to growth, has been called the growth apparatus. In long bones the growth apparatus is at the epiphyses or ends of the bones.
The length of the bone is controlled by the rate of growth of the epiphyseal line, which is the small cartilage plate at the end of the bone. Various stimuli have been implemented in trying to stimulate bone growth at the epiphyseal line. These stimuli have included periosteal irritation, radiation, medullary plugging, creation of an arteriovenous fistula, sympathetic denervation, heat, insertion of foreign objects into the epiphyses/metaphyses, and electricity. The methods of electrical stimulation have included electrolysis, i.e. the use of implanted dissimilar metals to produce a small current, direct current, electromagnetic fields and electric fields, both static and dynamic.
Efforts using electrolysis electrical stimulation have included inserting dissimilar metal strips into the metaphyses close to the growth plate on mongrel dogs, and creating a current of 10-20 microamperes between the two strips. However any positive results obtained were insignificant. Further work using bimetallic strips resulted in the stimulation of longitudinal growth, but the stimulus was unpredictable.
Other work has included that of Bassett who in 1974 showed that a capacitively coupled asymmetric electrostatic field increased the repair rate of fibular osteotomies in a rabbit. Watson in 1975 reported an increase in the length of embryonic chick tibiae grown in vitro in a pulsed square wave 1000 V/cm electric field. However, Watson obtained no positive results with a static field. Louis Norton in 1974 reported increased metabolic activity in the metatarsus bones of newborn chicks in response to a 5 Hz unidirectional signal ranging from 163 V to 490 V applied between two electrode capacitive plates. In 1977, Norton reported an increase in cAMP in response to a 900 V 5 Hz unidirectional signal, which increase tailed off at voltages above 1250 V. In 1976 Rodan and Norton demonstrated an enhanced incorporation of 3H-thymidine, in chondrocyte cultures, in response to a 1166 V/cm signal oscillating at 5 Hz. Similar results were reported by Norton working with membranous bone from rat calvaria in 1977.
In 1981, I reported electrical enhancement of growth plate DNA synthesis in vitro with low voltage capacitive coupling. In this experiment, costochondral junctions were excised from rats and were anchored to tightly sealed Petri dishes and grown in tissue culture medium. The Petri dishes were stimulated for 24 hours between intimately contacting, parallel, stainless steel plates using a stimulation signal of 10 V peak-to-peak, at a frequency of 60 KHz, with and without 50% amplitude modulation. Analysis of the samples indicated increased DNA synthesis of the experimental (stimulated) group relative to the control (unstimulated) group. However, the experiments were limited to in vitro specimens stimulated in Petri dishes.
In early 1982, I reported stimulation of in vitro epiphyseal plate growth by a time varying electric field using stimulation signals of 5-80 V peak-to-peak, frequencies between 30 and 120 KHz, and 0%, 50% and 100% modulation. Stimulatory effects of increased DNA synthesis was noted particularly over the range of 5-50 volts peak-to-peak and over a frequency range of 30-60 KHz. Again, this experiment was limited to in vitro specimens of costochondral junctions of rats stimulated in Petri dishes. While increased DNA synthesis was observed in this and the former experiment, both experiments were limited to in vitro specimens. Therefore, it was still not known whether certain stimulation signals would result in longitudinal bone growth at the epiphyseal growth plate in vivo, (i.e. in living bodies).